The Halloween Hypothesis
by MarieofIlledra
Summary: Sheldon enlists Penny's assistance to pull off a Halloween prank. Shenny fluffyness ensues!
1. Chapter 1

Penny blearily wandered into the boy's apartment in search of coffee, running into a whiteboard the second she opened the door.

"What the hell?" She managed to catch herself before she hit the floor and blinked in confusion, staring at the total destruction that surrounded her. The usually immaculate apartment was cluttered with old coffee cups, whiteboards, and crumpled bits of paper.

Sheldon rushed over, a cup of coffee in his hand. He thrust it at her distractedly, rescuing the whiteboard she had nearly knocked over.

Penny eyed the cup momentarily before shrugging and downing the coffee quickly. It was probably Sheldon's, but by the look in his eyes she would need the extra caffeine.

"Sweetie, what's wrong?"

He glared at the incoherent scribbles on the various whiteboards. "It appears that I am stuck."

Penny quickly grabbed another cup of coffee from the coffee maker. "Uh oh. Um…is there anything I can do to help? Or, um, should I call… Leonard? Or someone?"

His eyes lit up as he fixed his gaze suddenly on her. She took an instinctive step backwards as he sent her his best creepy-Joker-grin. "Actually, you _could_ help me with this. I am endeavoring to retaliate against the series of so called 'practical' jokes that Howard, Leonard, and Raj have been engaging in against me."

Penny grinned. "OOOOh,yes! You definitely need my help." She eyed the whiteboards with new interest. "You are getting bogged down on the technical details."

Sheldon squeaked as she deftly snatched the marker out of his hand and erased the closest white board.

"Now if you really want to screw with their heads, you need to give depth to their darkest fears…"

Two weeks later, the night of the University Halloween function, their plan began. Penny and Sheldon stood in the hallway, putting the finishing touches on their costumes.

Penny spun, modeling her full pink skirt and jeans jacket. She was dressed in what Sheldon had assured her was a screen accurate replica of Rose Tyler's costume in the Idiot's Lantern. The pink helmet swung jauntily from her wrist.

Sheldon really did look handsome in his suit, the brown and blue pinstripes showing off his lanky form. And his hair, while not the same as David Tennant's gravity defying mess, was a fair approximation.

"Ok, last chance. Are you sure you are comfortable with this plan?" Penny raised a challenging eyebrow. She had been waiting for him to back out of this since she had suggested it, and was impressed with how determined he had been to follow through with their caper.

Sheldon nodded confidently. "I will be in character, Penny. Although I do not usually engage in unsanitary activities such as hand holding and touching, David Tennant's portrayal of the Doctor is quite-" He stopped abruptly as Penny reached her hand out for his and he automatically recoiled.

"And you are certain you are not sick?" Sheldon queried to mask his hesitation.

Penny sighed. "One hundred percent. Please don't take my temperature again."

Sheldon took a deep breath and then grabbed her hand. "Excellent!" He gave her an only-slightly-manic smile. "Let's go emotionally scar my friends!"

Penny had to admit, she had never seen this side of Sheldon before. He had thrown himself whole heartedly into their ruse- although, now that she thought about it, he never really did anything half way. He had been holding her hand for almost…she checked her phone… two minutes, and although he was still sweating slightly more than usual he had actually relaxed his hand to the point where she could feel her fingers again.

She smiled proudly at him. "The boys are going to flip."

Sheldon laughed his weird breathy laugh. "Yes, this is going to skew their entire worldview and rip them out of their comfort zones, possibly forever."

Penny laughed. "Sometimes I worry about you, you know. Not now, but sometimes. You ready?"

Sheldon nodded, his grip on her hand tightening again to the point of almost pain. She winced and ran a hand down his arm in a soothing gesture. "You can do this sweetie, I promise. Anyone as smart as you can do anything you set your mind to, yeah?"

He relaxed. "Oh, of course. I was merely allowing my nerves to negatively impact me for a moment. You are right, this will be simple. Ready for the best bazinga ever?"

Sharing a conspiratorial smile, they walked indoors.


	2. Chapter 2

Leonard, Howard, and Raj stood in the middle of the room enjoying the snacks. "I hope Sheldon gets here soon, the longer he takes to get ready the more I worry about his costume." Leonard remarked.

Raj grinned. "Oh yes. Remember the one year that he invented his own lightsaber and dressed up as Luke? Only he hadn't tested it, and when he turned it on…"

Howard waved a hand dismissively. "He is fine now; he can use the hand and everything. I'm sure he wouldn't try anything like that again."

Nevertheless, they all stood on tiptoe and craned their heads nervously to the door. All the sudden Howard frowned slightly.

"Leonard, I thought you said Penny wasn't coming?"

He shrugged. "I invited her, but she said she had a date. Why?"

Howard motioned to the door. "Her date must have cancelled, she just walked in."

Leonard looked, but she had vanished. "Are you sure?" He was distracted by Sheldon's head appearing above the crowd, his normally tame hair unusually messy. "Here comes Sheldon."

Penny reached them first. "Hey guys!"

"Hello, Penny." Howard greeted.

"What are you wearing?" Leonard questioned. "You do know that this is supposed to be a costume party, right? And what happened to your date?"

Penny just smiled. "I'm wearing a costume, silly. And he's here somewhere. He's just gone to get us drinks."

All three of the boys' jaws dropped. Raj whispered something into Howard's ear. Howard nodded. "Penny, Raj wants to know if your date is someone who works at the university."

Penny smiled sympathetically. "Why else would I be at your university costume party on a date?"

Leonard gaped, apparently speechless.

Sheldon appeared with two drinks in hand. "Hello!" he greeted his friends. He carefully handed Penny one of them. "One Neuron Neutralizer for my favorite companion."

She smiled at him, trying not to laugh as his comment and their costumes registered on the group. "Thanks, sweetie." She grinned as he draped a casual arm around her shoulder, surprised by how relaxed he was.

Raj gaped and resumed his whispering to Howard. Leonard shook himself out of his stillness, face turning red.

"Um, Penny? I have a stupid question, but… Are you here with…Sheldon?"

Penny smiled mischievously. "Do you see anyone else dressed like the Doctor around here?"

Sheldon's eyes lit up. "Ooooh, I love this song. You know, Penny, the Doctor would say that the music in the 21st century is really underappreciated. This song, for instance…." His voice faded as he grabbed her hand and dragged her through the socializing scientists and on to the dance floor.

Leonard followed their progress, still dumbfounded. "Did you guys see that?"

Howard nodded. Raj cocked his head. "Did you see Penny and Sheldon on a date? And do you see them dancing now? Because I suspect that this is a dream."

Leonard finally moved from confusion into horror. "This isn't a dream, this is a nightmare!"

Penny smirked at the boy's horrified expressions as Sheldon tentatively slid his arms around her as they swayed on the dance floor. She was practically giddy with how well Sheldon was doing. And, not that she was admitting it out loud or anything, but she loved having license to touch him. Just for fun, she ran a hand up his arm. He didn't even appear to notice.

"Penny?"

"What's up, sweetie?"  
He glanced at the chandelier above them and she smiled as she saw him consider his answer before he tentatively answered.

"Assuming that that is, indeed, an invitation to tell you my original thought and not a legitimate question as to the location of the chandelier…" he waited for her slight nod of affirmation before continuing. "I find that I am enjoying this ruse immensely. Thank you for agree to participate in this 'practical joke' with me. I would not have been able to pull it off with someone I did not trust to the same extent."

Penny smiled, genuinely touched. "I'm enjoying myself too. And I'm glad that you feel comfortable enough with me to let me into your bubble."

He raised one eyebrow. "To what bubble are you referring?"

She patted his chest. "Don't worry about it, sweetie. Oh! Heads up! Howard and Raj are headed this way."

Sheldon nodded, immediately pulling her slightly closer. Raj and Howard lingered awkwardly behind them, dancing…or at least, she thought they were trying to dance.

Sheldon leaned closer to her. "Do you think we should pretend to be whispering 'sweet nothings' to each other?"

She caught his hands as they were halfway up to make the air quotes on 'sweet nothings'. She laughed flirtatiously, wrapping his arms back around her and pressing up against his chest flirtatiously. "Shel_don, _not here!"

Raj froze awkwardly in the middle of an arm flail, face going pale. He exchanged a glance with Howard, then abandoned his space on the dance floor and scurried back to Leonard's side.

Howard, unfortunately, was made of sterner stuff. "Okay, I seriously cannot think of one thing to say that will scare off _Howard._" Penny hissed, accidentally brushing his ear with her lips.

Sheldon blushed, and she giggled lightly as his hands reflexively gripped the fabric of her dress tightly. "Sorry, sweetie."

He swallowed hard. "Maybe we should get some air? It's hot in here."

He gripped her hand and wove expertly across the dance floor and out into the garden, leaving a slightly stunned Howard behind them.

They spent the rest of the function dancing and talking, amusing themselves not just with the boys' reactions but also the shock of Sheldon's co workers.

Sheldon was shocked to find that he had genuinely enjoyed himself. He was usually so stressed in social situations that he made himself miserable, but having Penny to guide him through social encounters meant that he could actually relax.

Sheldon glanced at his watch and gasped. "Oh! Penny, it's time for us to leave in order to complete the second phase of our plan!"

She grinned at him, only a little nervously. "Already? Wow, okay. Time flies when you are having fun, I suppose. Let me just run to the restroom."

He nodded distractedly, searching his pockets for her car keys. He dug out her phone, wallet, and extra lip gloss before locating her keys at the bottom of his pocket.

Dr. Gablehauser, who was standing nearby, snorted in amusement. "Amazing how many things women need us to carry for them when they dress up, isn't it?"

Sheldon smiled somewhat uneasily. He recognized this as a friendly gesture and guessed that his response was in some subtle way important. He nodded in agreement, stuffing Penny's supplies back into his pocket. "Indeed."

He couldn't think of another comment, but to his relief Penny was suddenly there, smiling proudly up at him as she took his arm.

"It's the price you men pay for us getting all dressed up in impractical clothing." She smiled brightly at Gablehauser, who chuckled good-naturedly.

"Suppose that's fair. Have a good night, Dr. Cooper." He winked at Sheldon as he wandered off in search of another drink.

Sheldon's brow furrowed. "He isn't usually that friendly towards me."

Penny smiled softly. "You know, people like you when you are acting human instead of condescending. Plus, he totally thinks you are getting lucky tonight."

Sheldon sputtered in shock but Penny ignored him, steering him swiftly out the door.  
"Come on, sweetie, we've got to get home before the boys do."


	3. Chapter 3

They were standing in the hallway waiting for Leonard, Howard, and Raj to arrive back at the apartment. Penny found that she was slightly nervous, which was ridiculous. It was just Sheldon. Phase one had been a resounding success, and phase two would be just as effective, she reassured herself.

Only, she was having trouble wrapping her mind around it. The plan was simple: when the boys came in, they would find Penny and Sheldon sharing a good night kiss in the hallway. Except that now that they were in the hallway, it was starting to feel a little awkward.

I mean, this was Sheldon. How was she supposed to kiss him in a believable fashion? And how would he manage to suppress his germophobia for long enough to actually kiss her? This was going to be awful.

Sheldon looked equally uncomfortable as he paced anxiously between their apartments.

"Penny-" He started, so suddenly that she jumped.

"Yes, Sheldon?"

"I'm concerned about the logistics of kissing you."

She blinked, unsure of his meaning. "Huh?"

"When we continue our ruse by letting Leonard, Raj, and Howard 'catch us' kissing." He used air quotes, and Penny immediately relaxed. This was just Sheldon, being Sheldon. She could handle this, no problem.

"Yeah, I remember the plan, sweetie. I just don't understand what logistics you are worried about."

He nodded, immediately going into lecture mode. "To begin with, there is a rather significant height difference between you and myself. And what if we both turn our heads the same direction, and instead of kissing we injure ourselves? And in a typical 'good night kiss', where is it appropriate for me to place my hands? How long should this kiss-" The longer he spoke the more agitated he became.

Penny held up a hand, cutting him off before he could get too panicky. "Woah, slow down. You are waaaay over-thinking this. It's all very natural, I promise."

Sheldon didn't look convinced, and she reluctantly decided that he was probably right. Sheldon's idea of what was natural was definitely slightly off from the average male.

She took in a deep breath and pasted a confident smile on her face. "Okay, come here." She led him over to her door and tugged at his arm until he placed his left hand against the wall next to her shoulder. "Keep that hand there," She instructed. He nodded obediently.

"When we kiss I'm going to go up on my toes, okay?" She put her hand lightly on her shoulder to stabilize herself and lifted up on her toes. "Is this okay?"

He nodded, looking much less freaked out than she had expected.

"Good. Okay, now you lean down a bit."

He smiled in relief as their eyes reached the same level. "Oh, good. That _does _fix the height issue. Do you normally lean your head to your left or your right when you kiss some one?"

She hesitated, tilting her head right and then left. "Uh, I'm not really sure. I've never paid attention. How about you decide what feels natural to you, and I'll adjust."

Sheldon twisted his neck oddly, staring intently at her face while he debated.

She stifled a giggle as the situation sank in. She never thought that she would _ever _have to give step by step kissing instructions to anyone. With anyone else in the world this would be beyond weird, and a total turn off, but teaching Sheldon _anything _was kind of fun.

And she wasa little curious to find out how he handled kissing. Maybe. A lot.

Sheldon gave up on his head tilting, looking dismayed. "How do I know which way feels natural? And where should I put my second hand? What type of kiss is first date appropriate?" He demanded.

Penny sighed. As fun as this way, it was going to take all night if he kept over thinking this. Throwing caution to the wind, she shifted her hand from his shoulder to the back of his neck and pressed her lips against his.

As she suspected, he froze completely. She mentally rolled her eyes as she released him and sank back on to her heels.

"What on earth was _that _for?" Sheldon hissed, twisting around as though he expected Leonard, Howard, and Raj to pop into existence behind him.

Penny smiled calmly. "You move your head right when you kiss someone. And you are going to have to participate a bit more if this is going to be believable."

He glared at her, taking a step backwards and crossing his arms defensively. "That is unfair! You…you…you took me by surprise."

She raised a challenging eyebrow. "Want to try again?" She offered before she could stop herself. He nodded, determined to get this right.

He gingerly placed his hand back on the wall beside her and nodded again, looking nervous.

She smiled reassuringly at him. "Just let me set the pace this time, ok?" She leaned in and her gaze dropped to his lips.

He mentally catalogued that as a warning sign that she was about to kiss him. Her eyes closed as she gently brushed their lips together.

His eyes fluttered shut as he considered the feeling. It was…different. Not unpleasant. He leaned in slightly towards her and she took the cue, bringing their lips more firmly together. She smiled slightly against his lips. He was doing really well, considering. He wasn't freaking out and running down the stairs at least.

Then, tentatively, he began to kiss her back. And, well, it was kind of amazing. He wasn't the world's best kisser, but there was something so ridiculously fantastic about the fact that this was _Sheldon. _Sheldon, who could barely handle physical contact from his best friends. Sheldon, who, unbelievably, was sliding his free hand into the hair at the base of her neck to change the angle of their kiss. She realized in surprise that she was the one who was frozen in shock this time.

Sheldon sighed slightly into the kiss and pulled away. "Penny, what are you doing?" He inquired.

She blushed. "Sorry, sweetie. Just surprised is all."

He smiled smugly at her. "I don't know why you would be surprised, this was the plan… and you are going to have to be much more convincing. Aren't you supposed to be a 'big ole five'?"

She rolled her eyes at his typical attitude. "Oh, I'll show you a big ole five." She muttered, grinning predatorily at him as she heard the lobby door open and the boys start up the stairs.

Sheldon considered, briefly, that it was probably not wise of him to antagonize her when he couldn't escape without destroying their ruse. And then her lips crashed against his and he was immediately caught up in her.


	4. Chapter 4

Leonard shook his head stubbornly as the three boys walked into the lobby. "No way. Penny would _never _go for Sheldon. I am telling you guys, this has got to be some sort of a joke."

Howard and Raj looked unconvinced, but began to climb the stairs. "I don't know, I just can't see either of them coming up with this as a joke. And you didn't hear them on the dance floor. It looked like things got kind of intense for a minute there." Howard combated.

Raj looked back at Leonard expectantly. Leonard didn't disappoint.

"No. Penny is the most touchy feel person I know, and Sheldon is afraid of physical contact. It would never happen. Trust me."

Raj hesitated. "I can see his point there. Penny is a sexual creature. And Sheldon is… well… not."

They all paused on the third landing to consider this thought.

Howard sighed, and they all started up the final flight of stairs. "I agree, but I also thought Sheldon didn't dance. And we all saw _that _happen tonight. I'm just saying…"

He came into sight of the landing and he trailed off. "Oh my god." Howard gasped.

Leonard tried to peer past him. "What?"

Howard tried to turn them all around, but it was too late. Raj and Leonard had rushed past him, and Penny and Sheldon were in plain view.

Sheldon had Penny shoved up against her apartment door, his tongue halfway down her throat. He had one hand in her hair, tilting her head back and kissing her passionately. His other hand was wrapped around her waist, holding her against his body. Penny, clearly, wasn't complaining. She clutched onto him as though he was the only thing holding her up and, as they stared in horror, she ran her nails down Sheldon's back, making him groan and press her farther against the door.

They didn't appear to notice the boys on the stairs. Leonard turned away, face pale.

"Oh, tell me that I'm dreaming." He begged. Howard shook his head.

"I don't think your subconscious would do this to you." He stated bluntly.

Raj stared in fascination. "Sheldon seems really into this. I thought you said he didn't have a deal?"

Leonard grabbed his friends. "Apparently we were terribly, horribly wrong. Let's go."

As they retreated hastily back down the stairs, Penny wondered how exactly she had wound up in this position.

When she initiated this kiss, she had been the one in control. She had used every technique she had, kissing Sheldon with a passion that she knew he was unprepared for. It had been worlds different from their previous kisses, and to be honest she had thought that he would immediately flip out.

She just hadn't thought that when he snapped he would be so damn hot. Instead of freaking out and pulling away, he had immediately seized control of the kiss. And damn. Could that boy kiss. All of the sudden, he was doing exactly the right things. He started out using things that she had tried on him, and then put his entire intellect into finding out what made her melt in his arms.

For example, he had already learned that when he tugged gently at her lower lip with his teeth that she would pull him closer and make a strangled sound in the back of her throat. It was almost as much fun as when he traced the shape of her top lip with his tongue. That was his favorite, he had decided, because she would instantly collapse against him, allowing him a few precious seconds of control before she recovered.

She shook her head dazedly as she dimly heard the lobby door slam shut. They should probably stop now, something whispered fuzzily in the back of her mind. She pulled back slightly from the kiss, gasping for air, fully intending to say something to that effect.

And then Sheldon took the opportunity to kiss her neck, right at her pulse. She shuddered, head falling back to give him more access.

He smiled against her neck. "I believe that phase two was also a resounding success." He murmured.

She shivered. Her neck was incredibly sensitive.

"Fascinating." He muttered. "I've heard that the neck is an erogenous zone before, but I never quite understood it." With every word he spoke his lips brushed against her skin, sending little jolts of pleasure down her body.

"Mmmhmm." This was unfair. He was new at this, she was supposed to be the one making _him _desperate. Smiling suddenly, she leaned up and placed a gentle kiss at the juncture of his collarbone and neck.

"Oh!" He gasped in surprise. She grinned teasingly, and licked gently at his pulse.

"Oh!" He repeated, clutching her closer.

She smiled wickedly against his skin before nipping gently, immediately soothing the spot with her tongue.

"Penny" He half whined, jumping about a foot in the air and groaning. She hesitated. Was it too much? "Do that again." He demanded suddenly, voice hoarse.

She obliged, this time biting a tiny bit harder and sucking gently. He would have a lovely hickey in the morning, if she had anything to say about it.

He groaned and gasped her name. She smiled. Yes, this was more like it. She kissed the spot lightly, and then all the sudden he was pulling her to his lips again.

This time the kiss was a little less frantic, a little more controlled. He was being deliberate, carefully cataloguing her likes and dislikes for future reference.

She gave herself up to the feeling of being wrapped inSheldon'sarms – _Sheldon's arms_ - and kissed him gently back.

She slowly ended the kiss. He sighed, reluctantly loosening his hold on her enough that she could stand on solid ground again. He waited until they were both capable of standing steadily and then let go of her altogether.

"I understand the appeal of kissing now." He stated, as calmly as he was able. As his voice was still husky with desire, he somewhat failed in his attempt at nonchalance.

Penny nodded. "Yup, pretty great, huh? I think Leonard, Howard, and Raj really bought it." She was afraid to look at him, worried that he had been only acting the part that he was supposed to. If so, he was one hell of an actor.

Her worries were made pointless when he gently lifted her head to look into her eyes. He licked his lips nervously.

"It occurs to me that this 'joke' of ours may not have exactly the results we wanted."

"What do you mean?"

"While I believe that we did rather thoroughly disturb our friend's worlds, I find myself wishing that I could kiss you again."

Penny beamed up at him. "Awww, sweetie. I think that could probably be arranged."

He hesitated, looking unsure if she was serious or not. "You would consent to add kissing to our friendship paradigm?"

She shook her head, and his heart fell. "No, I don't go around kissing my friends. Not like that, at least. You can kiss me whenever you want, but only if you are my boyfriend."

He met her gaze nervously. "And that would mean that you are my girlfriend? Penny I am not good at relationships! You know this."

She nodded seriously. "I know. But we will work it out, okay? Together. Really, not much will change."

"Except for kissing?" He confirmed. She smiled. He really had enjoyed that, hadn't he?

"Maybe not _just _kissing. But we will work up to that. For now, yeah. Kissing."

He swallowed hard. "I think I can live with that." He smiled shyly at her. "Can I kiss you again then?"

She laughed, wrapping her arms around his neck. "Of course, sweetie."

Their lips met once again, and she was just getting into the kiss when he abruptly stepped backwards, alarm evident on his face.

"Penny!" He looked horrified, and she immediately panicked.

"What is it?"

"We will have to come up with an entirely new practical joke! This will put us _weeks _behind on my plan to enact revenge on our friends!"

Penny laughed, pressing a gentle kiss to his cheek. "Don't worry, sweetie, I'm sure we can come up with something."

He nodded, looking reassured, and she pulled him into another kiss.

**The End! Thank you all SO much for reading! And especially to all of you who reviewed. **


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